As shown in a typical block diagram in FIG. 6, an LCD driver device 100 is composed of a voltage step-up circuit 1, a display voltage generating circuit 2, a panel driver 3, and a control circuit 4 built around a CPU or the like. The voltage step-up circuit 1 steps up a voltage VIN supplied from a battery 200 to VCC and outputs the stepped-up voltage. From the voltage VCC output from the voltage step-up circuit 1, the display voltage generating circuit 2 produces, for example, five display voltages V1, V2, V3, V4, and V5.
Using the plurality of display voltages V1, V2, V3, V4, and V5 output from the display voltage generating circuit 2, the panel driver 3 drives a plurality of common lines COM1, COM2, . . . , COMm provided in an LCD 300. Moreover, according to display data D fed from the control circuit 4 or from outside, the panel driver 3 drives a plurality of segment lines SEG1, SEG2, . . . , SEGn provided in the LCD 300.
As shown in FIG. 7, the LCD 300 has a plurality of common lines COM1, COM2, . . . , COMm and a plurality of segment lines SEG1, SEG2, . . . , SEGn arranged respectively at predetermined intervals to form a matrix in X and Y directions. At each intersection between the common lines COMx (X=1, 2, . . . , m) and the segment lines SEGY (Y=1, 2, . . . , n) is arranged a pixel P(x, y) having a liquid crystal layer, at one end of which is provided an electrode connected to the common line COMx and at the other end of which is provided an electrode connected to the segment line SEGY. Thus, depending on whether the voltage difference between the voltage applied to the electrode connected to the common line COMx and the voltage applied to the electrode connected to the segment line SEGY is greater than a threshold value or not, the pixel P(x, y) is turned either on or off.
According to commands and display data fed in by way of external signal lines S, the control circuit 4 controls the other circuits provided in the LCD driver device 100, and effects display. Specifically, when a command is fed in by way of the signal lines S to instruct the LCD 300 to start display, the control circuit 4 makes the voltage step-up circuit 1, the display voltage generating circuit 2, and the panel driver 3 start operating. On the other hand, when a command is fed in by way of the signal lines S to instruct the LCD 300 to stop display, the control circuit 4 makes the voltage step-up circuit 1, the display voltage generating circuit 2, and the panel driver 3 stop operating. Through this control, the voltage step-up circuit 1, the display voltage generating circuit 2, and the panel driver 3 are operated only when display on the LCD 300 is effected. This contributes to low electric power consumption. The control circuit 4 is kept all the time fed with, as the supply voltage from which it operates, the voltage VIN output from the battery.
Here, immediately after the start of operation, it takes time for the voltage step-up circuit to produce the stepped-up voltage, and it also takes time to charge the capacitors that are connected individually to the plurality of voltage lines of the display voltage generating circuit to smooth the display voltages and the parasitic capacitance present in each pixel. Therefore, the display voltages increase with finite gradients. Thus, in conventional LCD driver devices, it takes as long as 300 to 400 [mS] after the display voltage generating circuit starts operating until the display voltages reach the prescribed levels. Nevertheless, the panel driver starts operating the LCD at almost the same time that the display voltage generating circuit starts operating. Inconveniently, this results in disturbance of the displayed image immediately after display is started on the LCD.
The reason is that starting the driving of the LCD before the display voltages reach the prescribed levels hinders the voltage difference applied to each pixel of the LCD from settling at the prescribed value. As a result, pixels that should be turned on are left off, and pixels that should be kept off are turned on. This disturbance continues for 300 to 400 [mS], which is a period long enough to permit the human eye to perceive it. This period can be shortened by driving the display voltages with higher capacity, but this leads to increased current consumption.